Beverage preparation machines have been known for a number of years. For example, U.S. Pat. No. 5,943,472 discloses a water circulation system between a water reservoir and a hot water or vapour distribution chamber of an espresso machine. The circulation system includes a valve, metallic heating tube and pump that are connected together and to the reservoir via different silicone hoses, which are joined using clamping collars.
EP 1 646 305 discloses a beverage preparation machine with a heating device that heats circulating water which is then supplied to the inlet of a brewing unit. The brewing unit is arranged to pass heated water to a capsule containing a beverage ingredient for its brewing. The brewing unit has a chamber delimited by a first part and a second part movable relative to the first part and a guide for positioning a capsule in an intermediate position between the first and second parts before moving the first and second parts together from an open to a closed configuration of the brewing unit.
In-line heaters for heating circulating liquid, in particular water are also well known and are for example disclosed in CH 593 044, DE 103 22 034, DE 197 32 414, DE 197 37 694, EP 0 485 211, EP 1 380 243, FR 2 799 630, U.S. Pat. No. 4,242,568, U.S. Pat. No. 4,595,131, U.S. Pat. No. 5,019,690, U.S. Pat. No. 5,392,694, U.S. Pat. No. 5,943,472, U.S. Pat. No. 6,393,967, U.S. Pat. No. 6,889,598, U.S. Pat. No. 7,286,752, WO 01/54551 and WO 2004/006742.
More particularly, CH 593 044 and U.S. Pat. No. 4,242,568 disclose a coffee machine with an inline thermoblock heater having a metal mass with resistive heating cable cast in the mass and with a duct for the circulation of water to be heated.
Thermoblocks are in-line heaters through which a liquid is circulated for heating. They comprise a heating chamber, such as one or more ducts, in particular made of steel, extending through a (massive) mass of metal, in particular made of aluminium, iron and/or another metal or an alloy, that has a high thermal capacity for accumulating heat energy and a high thermal conductivity for the transfer the required amount of the accumulated heat to liquid circulating therethrough whenever needed. Instead of a distinct duct, the thermoblock's duct may by a through passage that is machined or otherwise formed in the duct's body, e.g. formed during a casting step of the thermoblock's mass. When the thermoblock's mass is made of aluminium, it is preferred, for health considerations, to provide a separate duct, for example of steel, to avoid contact between circulating liquid and aluminium. The block's mass can be made of one or several assembled parts around the duct. Thermoblocks usually include one or more resistive heating elements, for instance discrete or integrated resistors, that convert electrical energy into heating energy. Such resistive heating elements are typically in or on the thermoblock's mass at a distance of more than 1 mm, in particular 2 to 50 mm or 5 to 30 mm, from the duct. The heat is supplied to the thermoblock's mass and via the mass to the circulating liquid. The heating elements may be cast or housed into the metal mass or fixed against the surface of the metal mass. The duct(s) may have a helicoidal or another arrangement along the thermoblock to maximise its/their length and heat transfer through the block.
A drawback of thermoblocks lies in the difficulty to accurately control the temperature and optimise the required heating energy for bringing the liquid to be heated to the desired temperature. Indeed, the thermal inertia of the metal mass, the localised and uneven resistive heating of the mass, the dynamic heat diffusion from the heating in the mass to different parts of the mass affecting the measured temperature of the mass at predetermined locations make an accurate control of the thermoblocks to heat the circulating liquid to a desired predetermined temperature quite difficult and moreover requires quite long pre-heating periods, typically of 1 to 2 min in the case of espresso machines. Furthermore, it is difficult to predict various parameters involving the subsequent use of the thermoblock produced in series, e.g. the temperature of the environment, the net voltage of the mains, the actual value of the heating resistor of the thermoblock, thermal insulation of the thermoblock, the initial temperature of the liquid circulated through the thermoblock, etc. . . . Consequently, thermoblocks are usually associated with dynamic loop-controlled powering circuit tailoring the powering of the thermoblock with continuous measuring of the temperature. However, due to the complex thermal flow of such a system, the stabilisation of the thermoblock at a certain temperature level adjusted to the heating needs of the flow of liquid to be circulated is lengthy and still difficult to achieve.
An approach to improve the heating accuracy is taught in EP 1 380 243. This patent discloses a heating device intended in particular to equip coffee machines. This heating device comprises a metal tube through which the liquid that is to be heated can flow from an inlet duct to an outlet duct. The exterior surface of the tube is covered over several sections of its length with a plurality of sets of electric resistive elements in series. A cylindrical insert extends inside the tube to form, with the interior wall of the tube, a helical duct through which the liquid can circulate and which thus encourages turbulent flow and rapid transfer of energy from the tube to the liquid. A flowmeter is also positioned upstream of the inlet duct. The device further comprises a plurality of temperature sensors distributed along the length of the tube at the entry to and exit from each set of resistive elements. The principle governing the distribution of heating energy to the liquid in this instance is based on modulating the electrical power produced by the resistive elements which can be switched independently of one another or in series according to the water temperature at the inlet to the duct. Although this device gives results which are satisfactory in terms of the speed of heating, this device is relatively bulky in that the volume of water to be heated determines the height of the tube.
Furthermore, the accuracy with which the liquid temperature is regulated is limited by the fact that the liquid does not come into direct contact with the sensors which are positioned outside the tube. The rate of response to temperature differences, due to the inertia of the liquid that is to be heated, is also slower, and this detracts from the accuracy with which the temperature can be regulated. It should also be noted that the proximity of the temperature sensors to the sets of resistive elements runs the risk of influencing the measurement in an uncontrollable manner because of the thermal conduction that occurs through the wall of the tube.
In addition, more or less complex attempts to improve the thermal control of heaters for batch or in-line low inertia heaters have been proposed in DE 197 11 291, EP 1 634 520, U.S. Pat. No. 4,700,052, U.S. Pat. No. 6,246,831.
There is still a need to provide a simple and reliable heater for a fast pre-heating thereof for accurately heating a liquid circulated therethrough during normal use and under various conditions of use.